The invention relates to a method for producing a tempered, seamlessly hot-rolled steel pipe, in which a hollow block heated to forming temperature is rolled in a rolling mill to form a pipe with a finished diameter after rolling and is subsequently tempered with appropriate tempering parameters.
Since the Mannesmann brothers' invention for producing a thick-walled hollow block pipe from a heated block, various proposals have been made for stretching this hollow block pipe in the same heat in a further hot working stage, upon which the outer diameter is reduced to the finished diameter of the rolling mill.
Key words in relation to this are the continuous pipe rolling method, the push bench method, the Stiefel process and the Pilger process (Stahlrohr-Handbuch, 10th edition, published by Vulkan-Verlag Essen 1986, III. Herstellverfahren).
All of the named methods are preferred for different dimensional ranges and materials, wherein there is also some overlapping. For the middle dimensional range of 5″ to 18″ the continuous pipe rolling method and Stiefel process are used, for the dimensional range of up to 26″ the Pilger process is used. In the case of a thicker wall in the range of >30 mm the continuous pipe rolling method and Stiefel process are less suitable, while the Pilger process does not have any problems with the wall thickness but is slower in terms of production cycle.
In particular for the use of pipes for the petroleum oil or gas industry, DE 3127373 A1 discloses subjecting the pipes to a tempering treatment after reduction rolling to the finished diameter in order to achieve the required mechanical properties such as strength, durability and expansion. The tempering treatment itself consists, as is known, of heating to austenitization temperature, quenching and annealing.
It is also known that during this tempering treatment, grain growth takes place in the structure of the steel, which leads to an increase in the diameter of the pipe and must be taken into account with respect to the required finished diameter of the finished pipe after tempering.
Furthermore, the pipe expands during heating, and subsequent restriction of the shrinkage upon structural conversion, amongst other things, during the quenching process can also influence the diameter of the finished pipe.
For oil field and conduit pipes, requirements in terms of diameter tolerances, diameter ovality, wall thickness tolerances, weight per meter, driftability, etc., are imposed by standards, such as by the API (American Petroleum Institute) in dependence upon the intended use and dimensioning for the finished pipe.
These requirements lead to a situation where, for production purposes, the target diameter of the pipe after rolling is not always selected to be the same in the case of the same diameter being preset, e.g., by a standard, since this preset diameter is a compromise between production possibilities and production specifications. In addition, the pipe diameter grows more or less in dependence upon the material used and as a result of the change in grain size and the shrinkage restriction during tempering.
The simplest and most popular method for tackling this problem, in particular for diameters equal to or greater than 5½″ is to carry out a slight reduction in the diameter at annealing temperature with the aid of a sizing mill. This procedure is known, e.g., from JP 57155325 A or JP 2006307245 A. A sizing mill of this type usually has at least three stands in which the required finished diameters are produced after the pipes are tempered.
There are numerous disadvantages associated with these methods. In addition to the investment and operational costs for the sizing mill, energy consumption is higher since higher annealing temperatures are required for the sizing rolling so that a plastic deformation can take place in the sizing mill during the desired small reduction in diameter. The higher annealing temperatures also make it additionally necessary for the material to contain proportions of alloys in order to achieve the required mechanical-technological properties.
Alternatively, a pipe diameter adapted to the respective pipe after tempering could also be rolled in the rolling mill. However, this would lead to a considerably increased number of pipe diameters to be rolled and to a correspondingly large array of stands.